MGA Fuel Vaporisation

The MGA engine bay has it's issues. Not least the carburetor float chambers sit only an inch from the exhaust manifold, and the steel heat shield is painted in a dark colour which is poor for reflecting heat. Also the engine bay is very enclosed so air flow through the engine bay is poor and heat can build up.

My plan was to replace the old steel heatshield with a shiny aluminium one that might reflect a bit of heat.

You can see the proximity of the float chamber to the exhaust with the heatshield removed.

Modern car practice is to have heatshields right up against the exhaust manifolds, the idea being that heat is taken down the exhaust and released under the floor rather than in the engine bay.

But enclosing the exhaust manifold can increase the temperature towards the ignition point for oil and petrol, so wouldn't necessarily be a sensible option for a car with a track record of leaking both of these fluids around the engine bay.

I made the new heatshield slightly larger than the old one, and positioned it slightly closer to the exhaust.

The old heat shield made a good template for the hole positions which were drilled after all the folding and bending had been finished. I've also included an adjustable throttle linkage much like the one my bicycle brakes used to have.

I quite enjoyed making the heatshield so decided to knock a couple more up. The cunning idea was to fit the main heatshield on the engine side of the carburetor insulating blocks, and these further heatshields on the carb side to shield the float chambers.

Here the heatshields are installed, and I thought I'd test them out with a thermocouple.

After an enthusuiastic 5 mile drive and a long idle I opened up the bonnet. The heatshield closest to the exhaust was quite hot to the touch. The second heatshield was much cooler, and a the float chambers were 5 degrees C cooler than that.

Success - however well the new main heatshield is performing, the secondary ones are reducing fuel temperatures by 5 degrees C. That's got to help.

I thought I'd do some measurements. Here's a photo of the thermocouple installation. It's a cheap one that came with the volt meter that's taped to the wing. I could see the readings while driving.

The outside temperature was about 15 degrees C, and the float chamber stayed at around 35 degrees C when driving on the open road. After some stop start city driving the float chamber temperature slowly increased. At 45 degrees C the idle speed started to fall a little. The temperature rose to 49 degrees before I got bored and decided to investigate.

Turns out everything in the engine bay was at that temperature, and of course it would be - there's a great big radiator at the front heating everything up.

I fitted a manual switch to the engine cooling fan to see whether a constant airflow would help, but it didn't make any difference to the fuel temperature. I did wire the fan from live rather than ignition so the fan can cool the engine down while I'm buying my groceries, hopefully getting around the embarrassing hot starting problems.

The one thing that did help was a little fan from a computer directed from the cold air vents to the float chambers. That's my emergency backup plan, but I'll keep the car as it is for now, as I suspect the heatshields were the cause of the problem. Although I really wish I'd measured float chamber temperatures before I started.

The float chambers must have been reaching the boiling point of petrol (around 100 degrees C), so if I'm only seeing 50 degrees now then I've hopefully cured the problem.